GarageGuideTurbocharger/supercharger boost (underboost)
P0299 Code: Turbocharger/Supercharger Underboost
P0299 stands for "Turbocharger/Supercharger A Underboost Condition." A turbocharger uses exhaust flow to spin a compressor that pushes extra air into the engine, which lets a small engine make big power. The engine computer has a target boost pressure and measures what the turbo actually delivers. P0299 means actual boost stayed below the target.
Often the turbo itself is fine. Boost is pressurized air, and it can escape through a split hose or loose clamp on its way to the engine. Wastegate and control problems are also common. Turbo failure is possible, but it is usually not the first thing to suspect.
P0299 usually costs you power rather than leaving you stranded, and many cars protect themselves with limp mode. Sudden loss of power when merging can be unsafe, and a failing turbo can get worse quickly, so do not put it off.
You can usually drive gently. Avoid hard acceleration, towing, and merging situations where you need full power. If you see heavy smoke, hear a loud grinding or screeching from the turbo, or the oil level drops, stop and have it checked.
Symptoms you might notice
- Noticeable loss of power, especially when accelerating
- Car feels sluggish or goes into limp mode
- Hissing or whooshing sound under acceleration (often a boost leak)
- Check engine light on
- Worse fuel economy
- Sometimes blue or black exhaust smoke
Common causes
Boost leak
A cracked intercooler hose, a loose clamp, or a split intercooler lets pressurized air escape before it reaches the engine. This is one of the most common causes.
Wastegate stuck open or poorly controlled
The wastegate controls boost by letting exhaust bypass the turbo. If it sticks open, or its actuator or control solenoid fails, boost cannot build.
Dirty or restricted air filter
A clogged air filter or collapsed intake hose starves the turbo of air.
Faulty boost or MAP sensor
If the sensor misreads boost, the computer can set P0299 even with a healthy turbo.
Exhaust leak before the turbo
A leak at the exhaust manifold lets exhaust energy escape before it can spin the turbo.
Worn turbocharger
Worn bearings or a damaged compressor wheel reduce boost. Excess shaft play, oil in the intake, or a loud siren-like whine are clues.
Before you buy parts
P0299 does not automatically mean a bad turbo. Boost leaks from cracked hoses and loose clamps are among the most common causes and cost a fraction of a new turbocharger. Check the hoses, the air filter, and the wastegate first, and ask for a boost leak test before agreeing to a turbo replacement.
What to check
- Check the air filter and the intake hose between the filter and the turbo for clogs or collapse.
- With the engine off and cool, inspect the intercooler hoses and clamps (the large hoses between the turbo, intercooler, and engine) for cracks, oil residue, or loose clamps.
- Listen for hissing or whooshing under acceleration, which points to a boost leak.
- A shop can pressure-test the intake system (a boost leak test) to find leaks quickly.
- A scan tool shows desired versus actual boost and wastegate commands, which helps separate a leak from a wastegate or sensor problem.
Possible repairs
Repair a boost leak
Replacing a cracked intercooler hose or tightening a loose clamp is a common and affordable fix.
Replace the air filter
A clogged filter is cheap and easy to replace.
Repair the wastegate system
Replacing a failed wastegate actuator, control solenoid, or vacuum line restores boost control.
Replace a boost or MAP sensor
If the sensor reading is wrong, replacement fixes the false code.
Replace the turbocharger
If the turbo is worn, it is replaced, and the cause, often an oil supply problem, should be fixed so the new turbo lasts. This is usually a shop job.
Parts and tools that may help
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Frequently asked questions
Does P0299 mean my turbo is bad?
Not necessarily. A boost leak from a cracked hose or loose clamp, a wastegate problem, or a dirty air filter are common causes. The turbo itself should be checked last.
Can I drive with P0299?
Usually yes, gently. Avoid hard acceleration and towing. Stop if you see heavy smoke or hear loud grinding or screeching from the turbo.
What does a boost leak sound like?
Often a hiss, whoosh, or whistle when you accelerate, as pressurized air escapes from a hose or clamp.
What is a boost leak test?
A shop seals the intake and pressurizes it with air to find leaks by sound or with soapy water. It is the fastest way to find a leaking hose or intercooler.
How much does it cost to fix P0299?
It ranges from a clamp or hose to a wastegate part to a new turbocharger, which is the most expensive. Diagnose first to avoid replacing a good turbo.